Contaminated or spent acid may be produced in the course of a number of industrial processes such as refinery alkylation and the production of methylmethacrylates, methacrylic acid, and other monomer production processes.
The acid contaminants may include combustible species such as hydrocarbons and/or uncombustible species such as water and salt containing the sulfate (SO.sub.4.sup.-2).
The contaminated acid may be treated to produce fresh acid. One significant spent acid treatment process involves the use of heat to dissociate the acid, clean the dissociated products and then react them with oxygen followed by absorption of water to form fresh acid. For example, when the contaminated acid is sulfuric acid, the contaminated acid is introduced into a furnace along with fuel and air which is combusted to produce heat to vaporize the sulfuric acid and dissociate the sulfuric acid into water and sulfur dioxide. Both the vaporization and the dissociation of the sulfuric acid are endothermic. The resulting sulfur dioxide is passed, along with the other contents of the furnace, to a waste heat boiler to recover heat and then to a gas cleaning unit to produce clean sulfur dioxide. The clean sulfur dioxide is reacted with oxygen in a converter to produce sulfur trioxide and then passed through an absorption tower where the sulfur trioxide is reacted with water to produce fresh sulfuric acid.
A problem with this contaminated acid treatment system is that the processing rate of the furnace is less than the processing rate of the gas cleaning and the converter units. Moreover the heat generated within the furnace may not effectively vaporize and dissociate sufficient sulfuric acid to produce an adequate concentration of sulfur dioxide for effective downstream treatment. In this situation additional sulfur must be combusted in order to produce additional sulfur dioxide. This contributes substantially to the cost of the spent acid treatment.
The use of oxygen or oxygen-enriched air has been proposed for use in place of air as the oxidant in spent acid treatment systems since this would reduce the amount of inert nitrogen which could pass through the system and thus increase the overall processing rate of the furnace. However, such a simple substitution cannot be made because the oxygen or oxygen-enriched air and fuel will burn with a significantly increased peak flame temperature over that produced with air-fired combustion. These high peak flame temperatures lead to uneven heat distribution and hot spots within the furnace and furthermore kinetically favor the production of nitrogen oxides (NO.sub.x) which not only leads to environmental problems but also, in the case of sulfuric acid treatment, will cause contamination of final sulfuric acid product thus reducing its value.
Accordingly it is an object of this invention to provide a contaminated acid treatment method wherein oxygen or oxygen-enriched air may be effectively employed as the oxidant in a combustion reaction to provide heat for the endothermic dissociation of the contaminated acid.